In a steel mill, during the passage of the steel strip along the process line it is standard to measure the planarity of the strip being produced by a roller provided with force-measurement sensors set in the roller, forming measurement-point zones, and provided with covers for the force-measurement sensors. These covers are flush with the roller surface and are surrounded by a movement gap that allows some movement toward the respective force-measurement sensors.
Such planarity measurement rolls are known from EP 0,595,072 of Mucke in which the force-measurement sensors are spaced apart angularly and axially on the surface of the measurement roller so as to form respective measurement-point zones, with a free annular movement gap between the outer faces of their covers and the inner faces of the respective recesses in the roller in which they are housed (see EP 0,595,072). The annular movement gaps can each be closed by means of an O-ring or a plastic layer in order to prevent penetration of foreign particles with which the strips are often covered. These particles can be formed by abrasion of the zinc, so that in the case of zinc-coated strips the particles are of zinc. There is thus the risk of the particles wedging themselves in the movement gap and marring the strip surface as the strip passes in great longitudinal tension over the roller so that it bears with considerable radial force thereon.
In order to avoid such damage to the strip, it is known from German utility model 298 24 236 to provide the measurement roller and therefore its measurement-point zones with a covering. This covering can comprise several tubular sections or be a single cover tube. In the latter case, it is a thermally sprayed-on covering made from wear-resistant metal. A rubber coating or plastic coating, e.g. polyurethane coating, is however also known with planarity measurement rolls, in order to prevent foreign hard particles from collecting in the movement gaps of the coverings for the force-measurement sensors. The selection of a suitable coating or covering is dependent on the application. In the case of aluminum and special steel strips, for example, a rubber or plastic coating is usually used in order to take special care of the surface of the strip concerned. In the case of carbon-steel strip, on the other hand, hard coatings are often worked with, while with electrolytically zinc-coated strips a plastic coating is preferably employed.
If a hard coating of wear-resistant metals is deposited as a covering directly on the planarity measurement roll, there is a risk that, in the area of the movement gaps lying beneath this encasement, the hard coating will crack on account of microscopic movements of the coverings of the force-measurement sensors. This is because a radial elastic deformation of 1 μm to 2 μm due to the measurement force with a gap of 20 pm, for example, means 5 to 10% shear deformation. Such shear deformation cannot be absorbed for long by a hard coating.
If, on the other hand, the planarity measurement roller is surrounded by a support tube and the hard coating is deposited thereon the above-described shear deformations are avoided. Then, however, the auxiliary frictional connection via the tube has a disturbing effect, so that the measurement signals are negatively influenced in that the force must be applied through the tube to the sensors.